Most people are familiar with air filters used in their cars. These filters are essential to proper operation of the engine, and help extend the life of the engine and its components. The air available to the typical automotive or industrial combustion engine always includes some dirt and debris, or particulate material. Particulate material can cause substantial damage to the internal components of the particular combustion system if taken into the engine.
The function of the air intake filter is to remove the particulate matter from the intake air, so that clean air is provided to the engine. The intake air stream flows from the influent, or “dirty,” side of the filter to the effluent, or “clean,” side of the filter, with the air filter extracting the unwanted particles via one or more filter media layers. Filter media are selected to trap particles exceeding a particular size, while remaining substantially permeable to airflow over an expected filter lifetime.
The features and filter design choices that lead to improvements in one of these parameters can lead to losses in the other performance parameters. Thus, filter design involves trade-offs among features achieving high filter efficiency, and features achieving a high filter capacity and concomitant long filter lifetime.
As used herein, filter efficiency is the propensity of the filter media to trap, rather than pass, particulates. Filter capacity is typically defined according to a selected limiting pressure differential across the filter, typically resulting from loading by trapped particulates. Volumetric filter flow rate, or flow rate, is a measure of the volume of air that can be drawn into the filter having a particular effective filter area, efficiency, and capacity, at a particular point in the expected filter lifetime.
The choice of filter media that has a high filter efficiency (that is, it removes a high percentage of the particulate material in the intake air) is important, because any particulate matter passing through the filter may harm the engine. For systems of equal efficiency, a longer filter lifetime typically is directly associated with higher capacity, because the more efficiently a filter medium removes particles from an air stream, the more rapidly that filter medium approaches the pressure differential indicating the end of the filter medium life. To extend filter lifetime, filter media can be pleated, providing greater filtering surface area.
The choice of air filter media that is permeable to airflow is important because the interposition of the filter into the intake air stream can impede the flow rate. This tends to decrease engine efficiency, horsepower, torque, and fuel economy. In applications demanding large volumes of filtered air, the ability to manipulate parameters such as air filter size, pleat depth, or both, is often constrained additionally by the physical environment in which the filter operates, e.g., the space available for a filter of a given configuration within the engine compartment.
Some existing air filters dispense with filter housings or enclosures in order to expose as much air filter media for filtration as possible. Other existing air filters attempt to improve airflow by providing a filter pan, which is substituted for a conventional air filter housing cover.
However, in approaching high volumetric flow applications, existing filters that provide a significantly improved filter flow rate may foster air turbulence at the filter intake, an undesirable quality which ultimately impairs air flow. Some existing filter designs employ abrupt topological transitions, which tend to encourage air eddies to develop, and to reduce airflow into the filter. Exemplary abrupt transitions can include, for example, a one-step ring, ledge, edge, or peak. Because air eddies can cause the influent air to bypass regions of the filter media near these abrupt transitions, the effective area available for filtration is reduced.
Filters using multiple pieces also may be prone to allowing nuts, caps and attachment devices to fall into the engine air intake during maintenance. Such an event would be highly undesirable, for example, during a high-performance automotive race.
It is desirable, then, that an air filter effects both a minimal reduction in air flow as well as a minimal increase in the resistance, or restriction, to air flowing into the engine, while facilitating a large flow rate. It also is desirable that such an air filter be provided in a convenient and relatively economical physical configuration. The deleterious effects of certain harsh operating environments, such as construction sites, long haul operations and off-road, recreational, and sports applications, can lead to degraded efficiency or unacceptably short lifetimes in these air filters, especially under high airflow conditions. In some cases, existing high-efficiency air filters may not be capable of providing the desired volumetric flow rate under extreme conditions.